1. Related Application
This application claims priority to and the benefit of U.S. Provisional Application No. 60/655,905, filed Feb. 25, 2005, the entire contents of which is hereby incorporated by reference.
2. Field
The present invention relates to a lithographic apparatus and a method of making a device.
3. Background
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged, using a projection system, onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Development of new apparatus and methods in lithography have led to improvements in resolution of the imaged features, such as lines and contact holes or vias, patterned on a substrate, possibly leading to a resolution of less than 50 nm. This may be accomplished, for example, using relatively high numerical aperture (NA) projection systems (greater than 0.75 NA), a wavelength of 193 nm or less, and a plethora of techniques such as phase shift masks, non-conventional illumination and advanced photoresist processes.
However, certain small features such as contact holes are especially difficult to fabricate. The success of manufacturing processes at sub-wavelength resolutions will rely on the ability to print low modulation images or the ability to increase the image modulation to a level that will give acceptable lithographic yield.
Typically, the industry has used the Rayleigh criterion to evaluate the critical dimension (CD) and depth of focus (DOF) capability of a process. The CD and DOF measures can be given by the following equations:CD=k1(λ/NA),  (1)andDOF=k2(λ/NA2),  (2)where λ is the wavelength of the illumination radiation, k1 and k2 are constants for a specific lithographic process, and NA is the numerical aperture of the projection system.
Additional measures that provide insight into the difficulties associated with lithography at the resolution limit include the Exposure Latitude (EL), the Dense:Isolated Bias (DIB), and the Mask Error Enhancement Factor (MEEF). The exposure latitude describes the percentage dose range where the printed pattern's critical dimension (CD) is within acceptable limits. For example, the exposure latitude may be defined as the change in exposure dose that causes a 10% change in printed line width. Exposure Latitude is a measure of reliability in printing features in lithography. It is used along with the DOF to determine the process window, i.e., the regions of focus and exposure that keep the final resist profile within prescribed specifications. Dense:Isolated Bias is a measure of the size difference between similar features, depending on the pattern density. The MEEF describes how patterning device CD errors are transmitted into substrate CD errors. Other imaging factors that may be taken into account include the pitch. The pitch is a distance between two features such as, for example, contact holes. In a simplified approximation of coherent illumination, the resolution of a lithography system may also be quoted in terms of the smallest half-pitch of a grating that is resolvable as a function of wavelength and numerical aperture NA.
Among the trends in lithography is to reduce the CD by lowering the wavelength used, increasing the numerical aperture, and/or reducing the value of k1. However, increasing the numerical aperture would also lead to a decrease in the DOF which ultimately could lead to limitations in process latitude.
The loss in DOF with high NA is well known. However, the polarization targets for high NA partially coherent systems have not been thoroughly examined. According to the following equation:
                                          I            ⁡                          (                                                r                  →                                ;                                  z                  0                                            )                                =                                    ∑              i                        ⁢                                          ∫                S                            ⁢                                                ⅆ                                                            ρ                      →                                        0                                                  ⁢                                  S                  ⁡                                      (                                                                  ρ                        →                                            0                                        )                                                  ⁢                                                                                                                        FT                                                  -                          1                                                                    ⁢                                              {                                                                                                            O                              ~                                                        ⁡                                                          (                                                                                                ρ                                  →                                                                -                                                                                                      ρ                                    →                                                                    0                                                                                            )                                                                                ⁢                                                                                    P                              i                                                        ⁡                                                          (                                                              ρ                                →                                                            )                                                                                ⁢                                                                                    F                              i                                                        ⁡                                                          (                                                                                                ρ                                  →                                                                ;                                z                                                            )                                                                                ⁢                                                                                    H                              v                                                        ⁡                                                          (                                                                                                                                    ρ                                    →                                                                    ;                                                                      r                                    →                                                                                                  ,                                                                  z                                  0                                                                                            )                                                                                                      }                                                                                                  2                                                                    ⁢                                  ⁢                                            I              ⁡                              (                                  r                  ,                                      Z                    0                                                  )                                      =                                          ∑                i                            ⁢                                                ∫                  S                                ⁢                                                      ⅆ                                          ρ                      0                                                        ⁢                                      S                    ⁡                                          (                                              ρ                        0                                            )                                                        ⁢                                                                                                                                    FT                                                      -                            1                                                                          ⁢                                                  {                                                                                    O                              ⁡                                                              (                                                                  ρ                                  -                                                                      ρ                                    0                                                                                                  )                                                                                      ⁢                                                                                          P                                i                                                            ⁡                                                              (                                ρ                                )                                                                                      ⁢                                                                                          F                                i                                                            ⁡                                                              (                                                                  ρ                                  ,                                  z                                                                )                                                                                      ⁢                                                          H                              ⁡                                                              (                                                                  ρ                                  ,                                  r                                  ,                                                                      Z                                    0                                                                                                  )                                                                                                              }                                                                                                            2                                                                                ,                                    (        3        )            where the image intensity I, in a given film such as a photoresist, is a function of position {right arrow over (r)} and specific for a given focus position z0. This equation is valid for all values of numerical aperture NA and the image is the summation over all polarization states i. The integral is over the source distribution defined by S with source positions described by {right arrow over (ρ)}0. The inverse Fourier term within brackets represents the electric field distribution at the exit pupil. The four terms inside the bracket are, respectively, the object spectrum Õ of the pattern of the patterning device (e.g., pattern of the reticle), a polarization matrix P, a film matrix F and a pupil matrix Hv.
The pupil matrix is considered as the Jones pupil matrix. Generally, an optical element and/or polarizing element placed in a path of an input optical beam can change the output electromagnetic field distribution. This action is given by the following equation.
                                                                        E                output                            =                              JE                input                                                                                        =                                                [                                                                                                              J                          11                                                                                                                      J                          12                                                                                                                                                              J                          21                                                                                                                      J                          22                                                                                                      ]                                ⁡                                  [                                                                                                              E                          x                                                                                                                                                              E                          y                                                                                                      ]                                                                                                                        =                                  [                                                                                                              E                          x                          ′                                                                                                                                                              E                          y                          ′                                                                                                      ]                                            ,                                                          (        4        )            where J is known as the Jones matrix. In imaging theory, the values of E and J will be field dependent (given by the field vector, {right arrow over (r)}) but also dependent on the pupil position (given by pupil vector, {right arrow over (ρ)}). This gives rise to the lens pupils being described by a Jones matrix formalism.
Thus, following this formalism, the pupil matrix can be written as follows:
                                                                                          H                  v                                ⁡                                  (                                                            ρ                      →                                        ;                                          r                      →                                                        )                                            =                              [                                                                                                                              H                          xx                                                ⁡                                                  (                                                                                    ρ                              →                                                        ;                                                          r                              →                                                                                )                                                                                                                                                              H                          xy                                                ⁡                                                  (                                                                                    ρ                              →                                                        ;                                                          r                              →                                                                                )                                                                                                                                                                                                  H                          yx                                                ⁡                                                  (                                                                                    ρ                              →                                                        ;                                                          r                              →                                                                                )                                                                                                                                                              H                          yy                                                ⁡                                                  (                                                                                    ρ                              →                                                        ;                                                          r                              →                                                                                )                                                                                                                    ]                                                                                                        =                                                      H                    ⁡                                          (                                                                        ρ                          →                                                ;                                                  r                          →                                                                    )                                                        ·                                      [                                                                                                                                                      A                              xx                                                        ⁢                                                          ⅇ                                                              ⅈ                                ⁢                                                                                                                                  ⁢                                                                  δ                                  xx                                                                                                                                                                                                                                                        A                              xy                                                        ⁢                                                          ⅇ                                                              ⅈ                                ⁢                                                                                                                                  ⁢                                                                  δ                                  xy                                                                                                                                                                                                                                                                                                    A                              yx                                                        ⁢                                                          ⅇ                                                              ⅈ                                ⁢                                                                                                                                  ⁢                                δ                                ⁢                                                                                                                                  ⁢                                                                  y                                  xx                                                                                                                                                                                                                                                        A                              yy                                                        ⁢                                                          ⅇ                                                              ⅈ                                ⁢                                                                                                                                  ⁢                                                                  δ                                  yy                                                                                                                                                                                                          ]                                                              ,                                                          (        5        )            where the magnitude, A, the phases, δ, are explicit functions of pupil position {right arrow over (ρ)} and field position {right arrow over (r)}. A scalar pupil, H, has been factored out of the matrix elements. The scalar is given by:
                                          H            ⁡                          (                                                                    ρ                    →                                    ;                  r                                ,                                  z                  0                                            )                                =                                    A              ⁡                              (                                                      ρ                    →                                    ;                                      r                    →                                                  )                                      ⁢                          ⅇ                                                -                  ⅈ                                ⁢                                                                  ⁢                                  k                  0                                ⁢                                  z                  0                                ⁢                γ                                      ⁢                          ⅇ                                                -                  ⅈ                                ⁢                                                                  ⁢                                  k                  0                                ⁢                                  W                  ⁡                                      (                                                                  ρ                        →                                            ,                                              r                        →                                                              )                                                                        ⁢                                                            γ                  input                                γ                                                    ,                            (        6        )            where W describes the wave front aberrations, and γinput and γ are direction cosines at the entrance and exit pupils. Lenses that have birefringence will have pupil matrices defined by equation (5). The lens, in effect, will behave (within local pupil coordinates) as a polarization filter with representative Jones matrix. However, if the lens has aberration but no polarization behaviour then it is sufficient to use a simple scalar pupil function H.
The object intensity distribution is determined by the electric field at the plane of the patterning device. This form does not place restrictions on whether the field is based on a Kirchhoff representation or the more rigorous Maxwell equation solution of the electric field from a patterning device with finite features. For the Kirchhoff approximation, the illumination polarization will determine the x and/or y component amounts of the electric field. However, for the more rigorous calculation the spectrum is dependent on the interaction between the patterning device (e.g., reticle) topography and the illumination polarization in a non-trivial manner.
Hence, according to the general equation (3), high NA imaging is intrinsically linked with the polarization state and the thin film structure, where the electric field coupling and the power absorbed by a photoresist film can be drastically altered. As a result, the imaging properties on the resist can be altered which may include variations in linewidth, image contrast, and critical dimension (CD). The image contrast is a conventional image metric useful for small equal line/space patterns, the image contrast is defined as the difference between the maximum and minimum intensities in an image divided by the sum of maximum and minimum intensities.